This invention relates to a method for manufacturing diphenols and monoalkyl ethers and monoaryl ethers of diphenols having the hydrogen atom of one hydroxyl group thereof substituted with an alkyl group or aryl group by subjecting to oxidation with an organic per-acid corresponding phenols and alkyl ethers and aryl ethers of phenols having the hydrogen atom of the hydroxyl group thereof substituted with an alkyl group or aryl group.
Pyrocatechol and hydroquinone are highly useful compounds as antioxidants for various materials, polymerization inhibitors for monomers, medical and pharmaceutical stocks, spices, dyestuffs, intermediates for rubber stocks and photographic materials. Heretofore, pyrocatechol has been produced by a method which extracts the compound from the dry distillate of coal or a method which obtains the compound by hydrolyzing ortho-chlorophenol, for example. Hydroquinone has been produced by a method which obtains the compound by oxidizing aniline with manganese dioxide and sulfuric acid. The issues concerning the preservation of natural resources and the prevention of environmental pollution which have been brought to the public attention in the past years are encouraging a shift from existing unsafe processes to safe ones. There have been proposed many methods obtain pyrocatechol and hydroquinone by oxidizing phenols with hydrogen peroxide or with a per-acid. Methods which involve the oxidation of phenols with peracids have been disclosed by French Pat. No. 1,479,354 and by Ogata et al's article in the "Kogyo Kagaku Zasshi", Vol. 73, page 1849, published in 1970 by the Japan Chemical Society. And methods which involve the oxidation with hydrogen peroxide have been suggested by German Pat. No. 2,064,47, Japanese Patent Disclosure No. 34325/1972 and Japanese Patent Disclosure No. 30330/1974, for example.
The method which involves use of hydrogen peroxide as disclosed by German Pat. No. 2,064,497 is dangerous since the hydrogen peroxide to be used is required to have a high concentration of not less than 60 percent. The methods which permit use of a low-concentration aqueous solution of hydrogen peroxide require use of a metal salt (Japanese Patent Disclosure No. 28435/1973) and an organic metal chelate (Japanese Patent Disclosure No. 30330/1974) respectively as the catalyst. In the case of the method which involves use of the metal salt, said metal salt persists in the water phase during the extraction of the reaction product with an organic solvent and consequently passes into the waste water, entailing the possibility of water pollution. Thus, the process cannot be carried out in a closed system unless there is incorporated an extra facility for the removal of such unwanted metal. Otherwise, the water phase containing the metal salt in a dilute sate is concentrated to the extent of enabling the metal salt to be recovered and recycled in the process. This operation, therefore, calls for large equipment cost and heavy energy consumption. In the case of the method which involves use of the organic metal chelate compound as the catalyst, an organic solvent is used to extract the product aimed at from the reaction mixture. In the course of said extraction, the organic metal chelate compound dissolves into the organic phase and, consequently, in the subsequent course of distillation effected for separating the product from said organic phase, accelerates conversion of the product into tar. Moreover, the utility ratio of hydrogen peroxide falls short of 60 percent. Of the methods involving use of per-acids, the method disclosed by French Pat. No. 1,479,354 and the method proposed by Ogata et al in the Kogyo Kagaku Zasshi, Vol. 73, page 1849 synthesize the per-acids from aqueous solution of hydrogen peroxide and utilize the produced per-acids in situ. Thus, they are also dangerous since they involve use of an aqueous solution of not less than 60 percent of hydrogen peroxide. These methods, moreover, have the disadvantage that the yields are low, although use of such per-acids proves to be commercially advantageous in terms of the materials of which the production system is made.
For the production of monoalkyl ethers or monoaryl ethers of diphenols by the oxidation of alkyl ethers or aryl ethers of phenols with per-acids, there has been proposed a method which effects the oxidation by use of an organic per-acid of high reactivity in the presence of a catalyst such as of boron trifluoride, for example (J. D. McClure: J. Org. Chem., 27, 627 (1962)). This method is still without advantage because of low yields despite use of an expensive per-acid, because of inferiority in terms of materials used in the reaction system, etc. As concerns the oxidation of alkyl phenols with per-acids, no research directed to the synthesis of alkyl pyrocatechol has been brought to knowledge in the art.
One of the inventors of the present invention made a study in search of catalysts effective in oxidizing phenols with per-acids and arrived at the discovery that per-acid stabilizers, nitrogen group- or hydroxyl group-containing polycarboxylic acids possessed of an ability to chelate heavy metal ions, salts of said acids and acidic low esters of phosphoric acid and pyrophosphoric acid exhibit excellent catalytic activity (Japanese Patent Disclosure No. 86334/1974, No. 102631/1974, No. 134636/1974, No. 47934/1975, No. 49237/1975 and No. 76031/1975).
The catalysts to be used in the methods disclosed in said Japanese Patent disclosures exhibit inferior solubility in phenols or generally high boiling points, making it difficult to separate the catalysts from the formed products. Otherwise, they persist in the evaporators and consequently cause conversion of the formed products into tar. There is also a possibility that they decompose and give birth to various nitrogen-containing compounds, which possibly contaminate the end products and cause both internal and external contamination.
An object of the present invention is to provide a method for producing, without entailing the disadvantages attendant upon use of such known catalysts as described above, diphenols and monoalkyl ethers and monoaryl ethers of diphenols having the hydrogen atom of one hydroxyl group thereof substituted with an alkyl group or aryl group by subjecting to oxidation with an organic per acid the corresponding phenols and alkyl ethers and aryl ethers of phenols having the hydrogen atom of one hydroxyl group thereof substituted with an alkyl group or aryl group.